There is two different views on if a dog smiles. The expression that a dog makes is similar to that of a wolf. The expression we take as a smile indicates nervousness or submission to another wolf. Dogs are also hard-wired to interpret it in this way. But in reality, the "smile" is indicating that the dog is not threatening the state in which it was. You can find more information here: http://www.petplace.com/dogs/why-do-dogs-smile/page1.aspx
